The use of high-frequency electromagnetic energy as a means for heating polymers is an advancing art which finds application in many fields, especially in fields in which conventional conductive, convective, or radiant heat energy is either not suitable, not practical, or not possible. For instance, sealing of a polymer to itself or to some other substrate can be (if the composition of the polymer is appropriate) an important commercial technique in producing a desired end-product.
There are some polymers which are not suitable, or at least not well-suited, for high-frequency heating operations. In such ill-suited polymers high-frequency heating either does not occur, or if it occurs it does so only after inefficiently prolonged periods of treatment time. In production assembly lines, e.g., a quick heat-seal operation is generally preferred over a prolonged heat-seal operation.
Among the polymers which are ill-suited for high-frequency heating are olefin polymers, e.g., polymers and copolymers of ethylene, propylene, styrene or other .alpha.-olefinically unsaturated hydrocarbons having about 2 to about 10 carbon atoms. Some of these olefin polymers may contain polar groups, or polarizable groups, due to the incorporation therein of, e.g., acrylic (or methacrylic) acids or their alkyl esters as comonomers with the olefins, and these groups may, at high levels of incorporation tend to impart a modicum of high-frequency heatability to the polymer, but the efficacy is generally so slight that it is infeasible on a commercial basis. Some polymers having polar groups, e.g. chlorinated P.E., ethylene/vinyl acetate copolymer, PVC, polyvinylidene chloride, and polyamide, are heatable under the influence of certain frequencies of electromagnetic radiation, but are not generally well-suited for bonding using the higher frequencies.
Polyethylene is particularly known in the art to be substantially unsuitable for high-frequency heating unless sensitizers are added to the polymer; this is true regardless of whether it is linear or branched, or of whether it is low, medium, or high density (see, e.g., U.S. Pat. Nos. 3,336,173; 3,640,913; and 3,810,799).
It is an object of the present invention to provide a bonding film which has high-frequency heatability in order to bond a surface layer of material to a polymer foam substrate by the use of high frequency electromagnetic radiation.